Stack chip package structure

ABSTRACT

A stack chip package structure is provided. One principal feature of the structure is the formation of a few peripheral surfaces (e.g. ladder or lead-angle surfaces) at the bottom peripheral sections of a stack structure. When the stack structure is attached to a surface of a die through an adhesive layer, the thickness of the adhesive layer under a peripheral section of the stack structure is greater than a central region. Therefore, as the chip package is subjected to a thermal stress test, the adhesive layer under the peripheral sections of the stack structure is able to provide some buffering against thermal stress so that the stress concentration around the stack structure is reduced. Consequently, damages of the die surface due to stress are prevented and the average working life of the chip package is extended.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of Taiwan applicationserial no. 92201153, filed on Jan. 22, 2003.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a chip package structure. Moreparticularly, the present invention relates to a stack chip packagestructure.

2. Description of Related Art

As techniques for fabricating a smaller integrated circuit (IC)progresses, the level of integration inside an IC chip also increases.Moreover, the number of transistors inside the IC chip increases and thecross-sectional area of internal conductive lines are reduced. Ingeneral, heat is produced in each transistor and conductive line. Hence,a compact IC chip will produce a lot of heat during operation leading toan increase in temperature. As temperature of the IC chip rises to alevel above the normal operating temperature, computational errors,temporary malfunction or permanent damages may occur. Therefore, asidefrom providing suitable output signals via an interface, the IC chipmust be physically protected and have good heat dissipation capacity sothat the IC is maintained at a suitable operating temperature, and beprevented from exceeding the normal operating temperature range.

For most wire-bonding type of chip packages, a dummy die or a heatconductive metal block is stacked on top of a functional die to serve asa thermal conductive block. In this manner, the thermal impedance alongthe conductive path of the functional die is lowered. As a result, theheat generated by the chip is rapidly transferred to the outer surfaceof the chip package and dissipated away to the surrounding air.

FIG. 1 is a schematic cross-sectional view of a conventional stack chippackage structure. The package in FIG. 1 has a wire bonding (W/B) chippackage structure 100. The chip package structure 100 mainly comprises acarrier 110, a die 120, a thermal conductive block 130, an adhesivelayer 140, a plurality of conductive wires 150 and a molding compound160. The carrier 110 is, for example, a substrate or a lead frame (here,the carrier 110 is a substrate). The carrier 110 has a carrier surface112 and a plurality of bonding pads 114. The bonding pads 114 arepositioned on the carrier surface 112 of the carrier 110. In addition,the die 120 has an active surface 122 and a back surface 124. The backsurface 124 of the die 120 is attached to the carrier surface 112 of thecarrier 110 via the adhesive layer 142. The die 120 has a plurality ofmetal pads 126 on the active surface 122. The thermal conductive block130 is, for example, a dummy die or a metallic block with high thermalconductivity. The thermal conductive block 130 has a bonding surface 132that attaches to the active surface 122 of the die 120 through anotheradhesive layer 140. The metallic wires 150 connect each metal pad 126 onthe die 120 with a corresponding bonding pad 114 on the carrier 110electrically. The molding compound 160 encloses the die 120, the thermalconductive block 130 and the conductive wires 150.

As shown in FIG. 1, the thermal conductive block 130 has a rectangularstructure, for example. In other words, the bonding surface 132 (or thebottom surface) and a side surface 134 of the thermal conductive block130 form a right angle (the bonding surface 132 and the side surface 134form a 90° angle). This often leads to a problem of concentrating stressat the bottom peripheral sections of the thermal conductive block 130.When the chip package 100 is subjected to a thermal stress testing, forexample, a temperature cycle test (TCT) or a thermal shock test (TST),repeated heat expansion and cool contraction bends the thermalconductive block 130 and results a stress ring at its bottom peripheralsections that is particularly hard. As a result, a passivation layer(not shown) that covers the actives surface 122 of the die 120 may crackand damage some of the underlying wiring circuits (not shown).

In order to increase the elastic buffer between the die 120 and thethermal conductive block 130, the thickness of the adhesive layer 140may be increased. However, the thermal impedance of the adhesivematerial is much greater than the thermal impedance of the thermalconductive block material. Hence, the overall thermal impedance of thedie 120 is increased if thickness of the adhesive layer 140 isincreased. Therefore, to meet the thermal requirement of a particularpackage, there must be a maximal limitation for the thickness of theadhesive layer 140. Yet, the adhesive layer 140 may not be too thick tobuffer the passivation layer over the die 120 against repetitive stress.

SUMMARY OF INVENTION

Accordingly, one object of the present invention is to provide a stackchip package structure with a die and a thermal conductive block thereinsuch that stress concentration at the bottom peripheral sections of thethermal conductive block is reduced. Hence, the surface of the diesurface is subjected to a lower stress and the overall working life ofthe chip package is increased.

To achieve these and other advantages and in accordance with the purposeof the invention, as embodied and broadly described herein, theinvention provides a stack chip package structure. The stack chippackage at least includes a carrier, a die, an adhesive layer, and astack structure wherein the die is electrically connected with thecarrier. The die has a first surface and a second surface. The secondsurface of the die is corresponding to a carrier surface. The adhesivelayer is disposed on the first surface of the die. The stack structurehas a bonding surface. The bonding surface of the stack structure isattached to the first surface of the die via the adhesive layer. Thebonding surface further includes a central surface and at least oneperipheral surface. The peripheral surface surrounds the central surfaceand is further away from the first surface of the die than the centralsurface relatively. Therefore, the adhesive layer between one of theperipheral surfaces and the first surface is thicker than the adhesivelayer between the central surface and the first surface.

In brief, one major aspect of this invention is the production of atleast one peripheral surface at the bottom peripheral sections of astack structure. Hence, total amount of adhesive at the bottomperipheral sections of the stack structure is increased. When the chippackage is subjected to a thermal testing, the adhesive layer at thebottom section of the stack structure is able to provide some elasticbuffering so that stress at the bottom peripheral sections of the stackstructure is reduced considerably. Consequently, damages to the diesurface due to repeated thermal stress are prevented. Ultimately, theworking life of the chip package is extended.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a schematic cross-sectional view of a conventional stack chippackage structure.

FIG. 2A is a schematic cross-sectional view of a stack chip packagestructure according to a first preferred embodiment of this invention.

FIG. 2B is a schematic cross-sectional view of the chip packagestructure in FIG. 2A with a peripheral surface on the thermal conductiveblock, wherein the peripheral surfaces of the thermal conductive blockare multi-step ladder surfaces.

FIG. 2C is a schematic cross-sectional view of the chip packagestructure in FIG. 2A with a peripheral surface on the thermal conductiveblock, wherein the peripheral surfaces of the thermal conductive blockare sloping surfaces.

FIG. 2D is a schematic cross-sectional view of the chip packagestructure in FIG. 2A with a peripheral surface on the thermal conductiveblock, wherein the peripheral surfaces of the thermal conductive blockare curved surfaces.

FIG. 3 is a schematic cross-sectional view of a stack chip packagestructure according to a second preferred embodiment of this invention.

FIG. 4 is a schematic cross-sectional view of a stack chip packagestructure according to a third preferred embodiment of this invention.

FIG. 5 is a schematic cross-sectional view of a stack chip packagestructure according to a fourth preferred embodiment of this invention.

FIG. 6 is a schematic cross-sectional view of the chip package structurein FIG. 5 with a padding block inserted between two dies.

DETAILED DESCRIPTION

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

FIG. 2A is a schematic cross-sectional view of a stack chip packagestructure according to a first preferred embodiment of this invention,wherein the peripheral surfaces of the thermal conductive block aresingle ladder surfaces. As shown in FIG. 2A, the chip package structure200 mainly includes a carrier 210, a die 220, a thermal conductive block230, a adhesive layer 240, a plurality of conductive wires 250 and amolding compound 260. The carrier 210 is, for example, a substrate or alead frame (here, the carrier 210 is a substrate). The carrier 210 has acarrier surface 212 and a plurality of bonding pads 214 thereon. The die220 has an active surface 222 and a back surface 224. The back surface224 of the die 220 is attached to the carrier surface 212 of the carrier210 through an adhesive layer 242. The die 220 has a plurality of metalpads 226 on the active surface 222. The thermal conductive block 230 is,for example, a dummy die, a thermally conductive metallic block or agraphite block. The thermal conductive block 230 has a bonding surface232. The bonding surface 232 of the thermal conductive block 230 isattached to the active surface of the die 220 through the adhesive layer240. The adhesive layer 240 is fabricated using a material includingepoxy resin, for example. The conductive wires 250 connect each metalpad to a corresponding bonding pad 214 electrically. The moldingcompound 260 encloses the die 220, the thermal conductive block 230 andthe conductive wires 250.

As shown in FIG. 2A, the bonding surface 232 of the thermal conductiveblock 230 further includes a central surface 232 a and a plurality ofperipheral surfaces 232 b (in a magnified view of FIG. 2A). Note thatthe peripheral surfaces 232 b surround the central surface 232 a in adirection away from the active surface 222 of the die 220. Theperipheral surfaces 232 b and the central surface 232 a arenon-coplanar. Furthermore, the plurality of side surfaces 234 of thethermal conductive block 230 is indirectly connected to the centralsurface 232 a via the peripheral surfaces 232 b. Hence, the peripheralsurface 232 b is at a height level greater than the central surface 232a relative to the active surface 222. In other words, the peripheralsurface 232 b is further away from the active surface 222 when comparedwith the central surface 232 a.

After attaching the bonding surface 232 of the thermal conductive block230 to the active surface 222, the thickness of the adhesive layer 240between the peripheral surface 232 b and the active surface 222 isgreater than the adhesive layer 240 between the central surface 232 aand the active surface 222. Thus, thickness of the adhesive layer 240 atthe bottom peripheral sections of the thermal conductive block 230 isincreased. When the chip package structure 200 is subjected to a thermalstress test that includes cycles of expansion/contraction and bending,the adhesive layer 240 sandwiched between the peripheral surface 232 band the active surface 222 of the die 220 serves as an elastic buffer.In this way, stress concentration level at the bottom peripheralsections of the thermal conductive block 230 is lowered and damages ofthe die surface 222 due to stress are minimized.

FIG. 2B is a schematic cross-sectional view of the chip packagestructure in FIG. 2A with a peripheral surface on the thermal conductiveblock, wherein the peripheral surfaces of the thermal conductive blockare multi-step ladder surfaces. FIG. 2C is a schematic cross-sectionalview of the chip package structure in FIG. 2A with a peripheral surfaceon the thermal conductive block, wherein the peripheral surfaces of thethermal conductive block are sloping surfaces. FIG. 2D is a schematiccross-sectional view of the chip package structure in FIG. 2A with aperipheral surface on the thermal conductive block, wherein theperipheral surfaces of the thermal conductive block are curved surfaces.Aside from a single step in FIG. 2A, the peripheral surface 232 b can bea ladder with multiple steps as shown in FIG. 2B. Further, theperipheral surface 232 b also can be a sloping or a curved surface asshown in FIG. 2C and FIG. 2D. Therefore, the peripheral surface 232 b issimilarly capable of reducing the degree of stress concentration at thebottom peripheral sections of the thermal conductive block 230.

FIG. 3 is a schematic cross-sectional view of a stack chip packagestructure according to a second preferred embodiment of this invention.One major difference from the package in FIG. 2A is that the chippackage in FIG. 3 uses a leadframe instead of a substrate as a carrier310. With a lead-frame carrier 310, the carrier 310 has a die pad 310 aand a plurality of leads 310 b. A die 320 and a thermal conductive block330 are sequentially stacked over the die pad 310 a. One end of theleads 310 b provides a bonding pad 314 for attaching a conductive wire350 during a wire bonding process. Conductive wires 350 connect a metalpad 326 on the die 320 with the bonding pads 314 on the leads 310. Themolding compound 360 encloses the die 320, the thermal conductive block330, the conductive wires 360 and a portion of the carrier 310 (the diepad 310 and a portion of the leads 310 b). Note that the bonding surface332 of the thermal conductive block 330 is identical to the bondingsurface 232 in FIG. 2A. Both designs include a central surface (232 a)and a plurality of peripheral surfaces (232 b).

FIG. 4 is a schematic cross-sectional view of a stack chip packagestructure according to a third preferred embodiment of this invention.One major difference from the package in FIG. 2A is that the chippackage 400 in FIG. 4 is a type of lead on chip (LOC) package that hasno carrier. As shown in FIG. 4, instead of connecting through conductivewires 350 in FIG. 3, one end of each lead 411 is directly in contactwith a corresponding metal pad 426 on the active surface 422 of the die420. The bonding surface 432 of the thermal conductive block 430 issimilarly attached to the active surface 422 of the die 420 through anadhesive layer 440. In addition, a molding compound 460 encloses aportion of the die 420, the thermal conductive block 430 and a portionof the leads 411. Furthermore, the molding compound 460 may expose theback surface 424 of the die 420. Note that the bonding surface 432 ofthe thermal conductive block 430 has an identical design to the bondingsurface 232 shown in FIG. 2A. Both designs include a central surface(232 a) and a plurality of peripheral surfaces (232 b).

To form a system in package (SIP) module, a plurality of functional diesmay stack over a carrier. FIG. 5 is a schematic cross-sectional view ofa stack chip package structure according to a fourth preferredembodiment of this invention. As shown in FIG. 5, the chip packagestructure 500 mainly comprises a carrier 510, a first (functional) die520, a second (functional) die 570, a adhesive layer 540, a plurality ofconductive wires 550 and a molding compound 560. The chip packagestructure 500 uses a functional die 570 instead of a thermal conductiveblock (230). Metal pads 576 on the active surface 572 of the die 570 areelectrically connected to respective bonding pads 514 on the carriersurface 512 of the carrier 510 using conductive wires 550. Note that theback surface 574 of the die 570 has a configuration identical to thebonding surface 232 of the thermal conductive block 230 in FIG. 2A.Hence, the back surface 574 of the die 570 is attached to the activesurface 522 of the die 520 through the adhesive layer 540. With thisarrangement, thickness of the adhesive layer 540 at the bottomperipheral sections of the die 570 is greater than the one at the bottomcentral region of the die 570. The molding compound 560 encloses thefirst die 520, the second die 570, and the conductive wires 550.

FIG. 6 is a schematic cross-sectional view of the chip package structurein FIG. 5 with a padding block inserted between two dies. The chippackage structure 502 in FIG. 6 has a padding block 580 inserted intothe space between the lower die 520 and the upper die 570 to preventdirect contact or short-circuit of conductive wires 550 with the backsurface of the upper die 570. Note that the padding block 580 hasperipheral surfaces at both the upper peripheral sections and lowerperipheral sections so that the adhesive layer 540 in there is thickerthan in the central region. The non-uniformity of the adhesive layer 540provides some elastic buffering against thermal stress. In addition, thepadding block 580 can be a dummy die, a metal block, a graphite block orsome other material block.

Furthermore, each stack chip package according to this invention mayinclude a stack of stack structures (for example, functional dies, dummydies, metal blocks, graphite blocks or padding blocks). A plurality ofperipheral surfaces (e.g. ladder, sloping or curved surfaces) that maybe shaped at the bottom peripheral sections of one of the pair ofneighboring stack structures so that the adhesive layer is thickeraround the bottom peripheral sections than the central region to bufferagainst thermal stress.

In summary, this invention provides a stack chip package structure witha thicker adhesive layer around the edges than the central area of adie. In this manner thicker adhesive layer can be set near the edges ofa stack element (for example, a dummy die, a metal block, a graphiteblock or a functional die) simply by machining away a portion of thematerial from the edges to form a plurality of peripheral surfaces (e.g.ladder, sloping or curved surfaces). Hence, the design is able toincrease thickness of the adhesive layer in the edge region withoutthickening the adhesive layer elsewhere. When the chip package issubjected to a thermal testing, the adhesive layer at the bottom sectionof the stack structure is able to provide some buffering so that stressconcentration level at the bottom peripheral sections of the stackstructure is greatly reduced. Consequently, damages of the die surface(especially the active surface) due to repeated thermal stress areprevented and the working life of the chip package is increasedexpectedly.

Moreover, aside from a stack chip package structure with wire-bond (W/B)electrical connection, this invention can be applied to a package with alead on chip (LOC) or a system in package (SIP) design as well.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A stack chip package structure, comprising: a carrier with a carriersurface and a plurality of bonding pads, wherein the bonding pads areset up on the carrier surface; a die with an active surface and a backsurface, wherein the back surface of the die is in contact with thecarrier surface of the carrier and the active surface of the die has aplurality of metal pads thereon; an adhesive layer on the active surfaceof the die; a thermal conductive block with a bonding surface forattaching to the active surface of the die through the adhesive layer,wherein the bonding surface includes a central surface and a pluralityof peripheral surfaces surrounding the central surface, wherein theadhesive layer between at least one of the peripheral surfaces and theactive surface of the die is thicker than the adhesive layer between thecentral surface and the active surface, wherein the peripheral surfacesare multi-step ladder, curved or sloped surfaces; a plurality ofconductive wires electrically connecting each metal pad to acorresponding bonding pad; and a molding compound enclosing the die, thethermal conductive block and the conductive wires.
 2. The stack chippackage structure of claim 1, wherein the carrier is a substrate or alead-frame.
 3. The carrier is a lead-frame in claim 2 including: a diepad for stacking over sequentially a die and a thermal conductive block;and a plurality of leads having a bonding pad on one end of the leadsfor attaching a conductive wire.
 4. The stack chip package structure ofclaim 1, wherein the thermal conductive block is a dummy die, a metalblock or a graphite block.
 5. A stack chip package structure,comprising: a die with an active surface, wherein the active surface hasa plurality of metal pads thereon; an adhesive layer on the activesurface of the die; a thermal conductive block with a bonding surfacefor attaching to the active surface of the die through the adhesivelayer, wherein the bonding surface further includes a central surfaceand a plurality of peripheral surfaces surrounding the central surface,and the adhesive layer is between the central surface and the activesurface of the die and is between at least one of the peripheralsurfaces and the active surface of the die, wherein the peripheralsurfaces are further away from the active surface of the die than thecentral surface relatively, and the peripheral surfaces and the centralsurface are on non-coplanar planes and wherein the peripheral surfacesare multi-step ladder, sloped or curved surfaces; a plurality of leads,wherein each end of the lead is connected to a corresponding metal pad;and a molding compound for enclosing a portion of the die, the thermalconductive block and a portion of the leads.
 6. The stack chip packagestructure of claim 5, wherein the molding compound exposes the backsurface of the die.
 7. The stack chip package structure of claim 5,wherein a thickness of the adhesive layer between the peripheral surfaceand the active surface is greater than that of the adhesive layerbetween the central surface and the active surface.
 8. The stack chippackage structure of claim 5, wherein the thermal conductive block is adummy die, a metal block or a graphite block.
 9. A stack chip packagestructure, comprising: a die with a first surface; an adhesive layer onthe first surface of the die; and a stack structure with a bondingsurface for attaching to the first surface of the die, wherein thebonding surface further includes a central surface and a plurality ofperipheral surfaces surrounding the central surface and the adhesivelayer is between the central surface and the first surface of the dieand is between at least one of the peripheral surfaces of the stackstructure and the first surface of the die, wherein the peripheralsurfaces are further away from the active surface of the die than thecentral surface relatively, and the peripheral surfaces and the centralsurface are on non-coplanar planes, and the peripheral surfaces aremulti-step ladder, sloped or curved surfaces.
 10. The stack chip packagestructure of claim 9, wherein a thickness of the adhesive layer betweenthe peripheral surface and the active surface of the die is greater thanthe adhesive layer between the central surface and the active surface ofthe die.
 11. The stack chip package structure of claim 9, wherein thestack structure includes a dummy die.
 12. The stack chip packagestructure of claim 9, wherein the stack structure includes a thermalconductive block.
 13. The stack chip package structure of claim 9,wherein the stack structure includes a functional die.
 14. The stackchip package structure of claim 9, wherein the first surface of the diehas a plurality of metal pads and the stack chip package structurefurther includes: a carrier with a carrier surface and a plurality ofbonding pads, wherein the bonding pads are set up on the carrier surfaceand a second surface of the die is attached to the carrier surface ofthe carrier; a plurality of conductive wires for connecting each metalpad with a corresponding bonding pad electrically; and a moldingcompound for enclosing the die, the stack structure and the conductivewires.
 15. The stack chip package structure of claim 14, wherein thecarrier is a substrate or a lead-frame.